The rates and position of cleavage by trypsin of the larger of two proteins in (Na ion plus K ion)-ATPase, an enzyme responsible for cation transport in cell membranes are dependent on ion and ATP-induced conformational changes. Occupancy of protein sulfhydryl groups by N-ethylmaleimide blocks the ATP effect. The smaller glycoprotein, normally resistant to trypsin, is rapidly hydrolyzed when the enzyme is in a catalytically inactive conformation induced by potassium, magnesium and ATP. BIBLIOGRAPHIC REFERENCE: Takeguchi, C.A., Honegger, U.E., Holland, W.W. and Titus, E.O.: Evidence for subclasses of SH groups in (Na ion plus K ion)-ATPase. LifeSci. 19: 797-806,1976.